This invention relates to a method of preparing substituted aromatic carboxylic acid compounds. In particular, the present invention relates to liquid phase reaction of halogen-substituted ortho-xylene to produce halophthalic acid and halophthalic anhydride.
Oxidation of dialkyl benzenes has long been used to produce dicarboxylic acids. The oxidation has also been carried out in liquid phase in presence of a solvent. Of particular interest has been the oxidation of dimethyl benzene (xylene) to phthalic acid and the oxidation of para-xylene to terephthalic acid, which is in turn employed in the production of polybutylene terephthalate. Various methods for oxidizing ortho-xylene are known. For example, U.S. Pat. No. 3,402,184 describes oxidation of ortho-xylene in acetic acid solvent in the presence of a bromine promoter. U.S. Pat. Nos. 5,958,821; 5,981,420; and 6,020,522 describe oxidation of ortho-xylene in acetic acid solvent in the presence of a hydroxyimide promoter. Methods for preparing 4-chlorophthalic anhydride are also known. However, these methods typically involve aromatization of a Diels-Alder adduct of chloroprene and a maleic anhydride as in U.S. Pat. No. 5,322,954, or chlorination of phthalic acid as in Japanese patent applications 07258152 and 02129143. The chlorination process may also produce highly undesirable polychlorinated biphenyls. There is a need for a method for producing 4-chlorophthalic anhydride which does not involve handling toxic chloroprene or chlorine gas.
The liquid phase oxidation of xylene to phthalic acid requires the use of a catalyst, typically a cobalt/manganese/bromide catalyst system, and is generally performed in a carboxylic acid solvent such as acetic acid. The catalyst system may be augmented by the use of a co-catalyst such as zirconium, hafnium or cerium. Phthalic acid is an easily isolable solid, which can be filtered out of the reaction mixture.
Liquid phase oxidation, using a cobalt/manganese/bromide catalyst system and a carboxylic acid solvent, has also been applied to halogenated xylene with some success. The oxidation of the halogenated xylene is, however, more difficult than the oxidation of xylene due to presence of a halogen, which is an electron withdrawing substituent, on the benzene ring. The greater difficulty in oxidation results in a lower reaction selectivity and a larger amount of partial oxidation and side products than seen in the liquid phase oxidation of xylene under similar conditions. Additionally, the relatively soluble halogenated phthalic acid is difficult to separate from the partial oxidation and side products. Thus it is clear that in order for a method of halogenated xylene liquid phase oxidation to be successful, the reaction yield and the reaction selectivity must be very high and by-product formation should be minimized. Furthermore, in order to improve the commercial attractiveness of the liquid phase oxidation of halogenated xylene, effective methods for recovering high purity acetic acid (ex., up to 99.9 weight percent purity and having less than 0.01 weight percent of HCl) for reuse in the oxidation reaction are also desirable.